FRIDAY, May 23, 2014 (HealthDay News) -- Serious mental illness can take between seven and 24 years off a person's life, which is similar to or worse than the impact of heavy smoking, researchers report.

"We found that many mental health diagnoses are associated with a drop in life expectancy as great as that associated with smoking 20 or more cigarettes a day," Dr. Seena Fazel, of the department of psychiatry, at the University of Oxford in the United Kingdom, said in a university news release.

"There are likely to be many reasons for this. High-risk behaviors are common in psychiatric patients, especially drug and alcohol abuse, and they are more likely to die by suicide," Fazel noted. "The stigma surrounding mental health may mean people aren't treated as well for physical health problems when they do see a doctor."

Fazel's team examined 20 studies that looked at the link between mental illness and death rates. The studies included more than 1.7 million people, and 250,000 deaths.

The researchers found that major mental disorders can greatly shorten people's lives. For example, the average life expectancy was 10 to 20 years shorter than normal for people with schizophrenia, 9 to 20 years shorter for those with bipolar disorder, 7 to 11 years shorter for those with recurrent depression, and 9 to 24 years shorter for people with drug and alcohol abuse.

By contrast, heavy smoking shortens life by an average of 8 to 10 years, the study authors noted.

Although this study found an association between mental illness and an increased risk of premature death, it did not prove that mental illness causes early death.

While mental illness appears to greatly increase the risk of premature death, it's not a major public health priority, the researchers noted. One of the reasons is the tendency to separate mental and physical health.

"Many causes of mental health problems also have physical consequences, and mental illness worsens the prognosis of a range of physical illnesses, especially heart disease, diabetes and cancer. Unfortunately, people with serious mental illnesses may not access health care effectively," Fazel said in the news release.

He and his colleagues said their findings should push governments and health officials to place a much higher priority on mental health.

Findings from the study were released online May 23 in the journal World Psychiatry.